path: root/docs/CodingStandards.rst

.. _coding_standards:

=====================
LLVM Coding Standards
=====================

.. contents::
   :local:

Introduction
============

This document attempts to describe a few coding standards that are being used in
the LLVM source tree.  Although no coding standards should be regarded as
absolute requirements to be followed in all instances, coding standards are
particularly important for large-scale code bases that follow a library-based
design (like LLVM).

This document intentionally does not prescribe fixed standards for religious
issues such as brace placement and space usage.  For issues like this, follow
the golden rule:

.. _Golden Rule:

    **If you are extending, enhancing, or bug fixing already implemented code,
    use the style that is already being used so that the source is uniform and
    easy to follow.**

Note that some code bases (e.g. ``libc++``) have really good reasons to deviate
from the coding standards.  In the case of ``libc++``, this is because the
naming and other conventions are dictated by the C++ standard.  If you think
there is a specific good reason to deviate from the standards here, please bring
it up on the LLVMdev mailing list.

There are some conventions that are not uniformly followed in the code base
(e.g. the naming convention).  This is because they are relatively new, and a
lot of code was written before they were put in place.  Our long term goal is
for the entire codebase to follow the convention, but we explicitly *do not*
want patches that do large-scale reformating of existing code.  On the other
hand, it is reasonable to rename the methods of a class if you're about to
change it in some other way.  Just do the reformating as a separate commit from
the functionality change.
  
The ultimate goal of these guidelines is the increase readability and
maintainability of our common source base. If you have suggestions for topics to
be included, please mail them to `Chris <mailto:sabre@nondot.org>`_.

Mechanical Source Issues
========================

Source Code Formatting
----------------------

Commenting
^^^^^^^^^^

Comments are one critical part of readability and maintainability.  Everyone
knows they should comment their code, and so should you.  When writing comments,
write them as English prose, which means they should use proper capitalization,
punctuation, etc.  Aim to describe what the code is trying to do and why, not
*how* it does it at a micro level. Here are a few critical things to document:

.. _header file comment:

File Headers
""""""""""""

Every source file should have a header on it that describes the basic purpose of
the file.  If a file does not have a header, it should not be checked into the
tree.  The standard header looks like this:

.. code-block:: c++

  //===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===//
  //
  //                     The LLVM Compiler Infrastructure
  //
  // This file is distributed under the University of Illinois Open Source
  // License. See LICENSE.TXT for details.
  //
  //===----------------------------------------------------------------------===//
  //
  // This file contains the declaration of the Instruction class, which is the
  // base class for all of the VM instructions.
  //
  //===----------------------------------------------------------------------===//

A few things to note about this particular format: The "``-*- C++ -*-``" string
on the first line is there to tell Emacs that the source file is a C++ file, not
a C file (Emacs assumes ``.h`` files are C files by default).

.. note::

    This tag is not necessary in ``.cpp`` files.  The name of the file is also
    on the first line, along with a very short description of the purpose of the
    file.  This is important when printing out code and flipping though lots of
    pages.

The next section in the file is a concise note that defines the license that the
file is released under.  This makes it perfectly clear what terms the source
code can be distributed under and should not be modified in any way.

The main body of the description does not have to be very long in most cases.
Here it's only two lines.  If an algorithm is being implemented or something
tricky is going on, a reference to the paper where it is published should be
included, as well as any notes or *gotchas* in the code to watch out for.

Class overviews
"""""""""""""""

Classes are one fundamental part of a good object oriented design.  As such, a
class definition should have a comment block that explains what the class is
used for and how it works.  Every non-trivial class is expected to have a
``doxygen`` comment block.

Method information
""""""""""""""""""

Methods defined in a class (as well as any global functions) should also be
documented properly.  A quick note about what it does and a description of the
borderline behaviour is all that is necessary here (unless something
particularly tricky or insidious is going on).  The hope is that people can
figure out how to use your interfaces without reading the code itself.

Good things to talk about here are what happens when something unexpected
happens: does the method return null?  Abort?  Format your hard disk?

Comment Formatting
^^^^^^^^^^^^^^^^^^

In general, prefer C++ style (``//``) comments.  They take less space, require
less typing, don't have nesting problems, etc.  There are a few cases when it is
useful to use C style (``/* */``) comments however:

#. When writing C code: Obviously if you are writing C code, use C style
   comments.

#. When writing a header file that may be ``#include``\d by a C source file.

#. When writing a source file that is used by a tool that only accepts C style
   comments.

To comment out a large block of code, use ``#if 0`` and ``#endif``. These nest
properly and are better behaved in general than C style comments.

``#include`` Style
^^^^^^^^^^^^^^^^^^

Immediately after the `header file comment`_ (and include guards if working on a
header file), the `minimal list of #includes`_ required by the file should be
listed.  We prefer these ``#include``\s to be listed in this order:

.. _Main Module Header:
.. _Local/Private Headers:

#. Main Module Header
#. Local/Private Headers
#. ``llvm/*``
#. ``llvm/Analysis/*``
#. ``llvm/Assembly/*``
#. ``llvm/Bitcode/*``
#. ``llvm/CodeGen/*``
#. ...
#. ``llvm/Support/*``
#. ``llvm/Config/*``
#. System ``#include``\s

and each category should be sorted by name.

The `Main Module Header`_ file applies to ``.cpp`` files which implement an
interface defined by a ``.h`` file.  This ``#include`` should always be included
**first** regardless of where it lives on the file system.  By including a
header file first in the ``.cpp`` files that implement the interfaces, we ensure
that the header does not have any hidden dependencies which are not explicitly
``#include``\d in the header, but should be. It is also a form of documentation
in the ``.cpp`` file to indicate where the interfaces it implements are defined.

.. _fit into 80 columns:

Source Code Width
^^^^^^^^^^^^^^^^^

Write your code to fit within 80 columns of text.  This helps those of us who
like to print out code and look at your code in an ``xterm`` without resizing
it.

The longer answer is that there must be some limit to the width of the code in
order to reasonably allow developers to have multiple files side-by-side in
windows on a modest display.  If you are going to pick a width limit, it is
somewhat arbitrary but you might as well pick something standard.  Going with 90
columns (for example) instead of 80 columns wouldn't add any significant value
and would be detrimental to printing out code.  Also many other projects have
standardized on 80 columns, so some people have already configured their editors
for it (vs something else, like 90 columns).

This is one of many contentious issues in coding standards, but it is not up for
debate.

Use Spaces Instead of Tabs
^^^^^^^^^^^^^^^^^^^^^^^^^^

In all cases, prefer spaces to tabs in source files.  People have different
preferred indentation levels, and different styles of indentation that they
like; this is fine.  What isn't fine is that different editors/viewers expand
tabs out to different tab stops.  This can cause your code to look completely
unreadable, and it is not worth dealing with.

As always, follow the `Golden Rule`_ above: follow the style of
existing code if you are modifying and extending it.  If you like four spaces of
indentation, **DO NOT** do that in the middle of a chunk of code with two spaces
of indentation.  Also, do not reindent a whole source file: it makes for
incredible diffs that are absolutely worthless.

Indent Code Consistently
^^^^^^^^^^^^^^^^^^^^^^^^

Okay, in your first year of programming you were told that indentation is
important.  If you didn't believe and internalize this then, now is the time.
Just do it.

Compiler Issues
---------------

Treat Compiler Warnings Like Errors
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If your code has compiler warnings in it, something is wrong --- you aren't
casting values correctly, you have "questionable" constructs in your code, or
you are doing something legitimately wrong.  Compiler warnings can cover up
legitimate errors in output and make dealing with a translation unit difficult.

It is not possible to prevent all warnings from all compilers, nor is it
desirable.  Instead, pick a standard compiler (like ``gcc``) that provides a
good thorough set of warnings, and stick to it.  At least in the case of
``gcc``, it is possible to work around any spurious errors by changing the
syntax of the code slightly.  For example, a warning that annoys me occurs when
I write code like this:

.. code-block:: c++

  if (V = getValue()) {
    ...
  }

``gcc`` will warn me that I probably want to use the ``==`` operator, and that I
probably mistyped it.  In most cases, I haven't, and I really don't want the
spurious errors.  To fix this particular problem, I rewrite the code like
this:

.. code-block:: c++

  if ((V = getValue())) {
    ...
  }

which shuts ``gcc`` up.  Any ``gcc`` warning that annoys you can be fixed by
massaging the code appropriately.

Write Portable Code
^^^^^^^^^^^^^^^^^^^

In almost all cases, it is possible and within reason to write completely
portable code.  If there are cases where it isn't possible to write portable
code, isolate it behind a well defined (and well documented) interface.

In practice, this means that you shouldn't assume much about the host compiler
(and Visual Studio tends to be the lowest common denominator).  If advanced
features are used, they should only be an implementation detail of a library
which has a simple exposed API, and preferably be buried in ``libSystem``.

Do not use RTTI or Exceptions
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In an effort to reduce code and executable size, LLVM does not use RTTI
(e.g. ``dynamic_cast<>;``) or exceptions.  These two language features violate
the general C++ principle of *"you only pay for what you use"*, causing
executable bloat even if exceptions are never used in the code base, or if RTTI
is never used for a class.  Because of this, we turn them off globally in the
code.

That said, LLVM does make extensive use of a hand-rolled form of RTTI that use
templates like `isa<>, cast<>, and dyn_cast<> <ProgrammersManual.html#isa>`_.
This form of RTTI is opt-in and can be added to any class.  It is also
substantially more efficient than ``dynamic_cast<>``.

.. _static constructor:

Do not use Static Constructors
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Static constructors and destructors (e.g. global variables whose types have a
constructor or destructor) should not be added to the code base, and should be
removed